The invention relates to drug delivery systems by which set doses may be apportioned from a cartridge and delivered, the dose setting being made by changing the relative position of cooperating dose setting elements and the delivery being made by pressing a button until it abuts a stop.
The deliverance may take place as an aerosol spray, an injection, or a high pressure jet which penetrates the skin so that the delivered material may be precipitated in the tissue beneath the skin.
The dose setting elements may be a threaded rod and a nut as in EP 327 910 wherein the threaded rod is a piston rod acting on a piston which presses out liquid from a cartridge in accordance with the distance the piston is moved. By dose setting a nut at the end of a hollow button surrounding a threaded piston rod is screwed along the piston rod away from a stop and thereby lifts the button up from the proximal end of the injection device. The injection is performed by pressing the button to move the nut back to the stop by which movement the piston rod and the piston is pressed into the cartridge a distance corresponding to the distance the nut was screwed away from the stop.
In another dose setting mechanism as described in U.S. Pat. No. 4,865,591 a carrier, which may transport the piston rod in only a distal direction, is by the setting of a dose moved away from a stop and may thereafter by pressing an injection button be moved back to abutment with said stop.
In still another injection device as described in EP 0 245 312 a button with a push rod may be operated to reciprocate over a fixed distance. During part of its reciprocating movement the push rod drives a piston into a cartridge. Said part corresponds to a set dose which is set by varying a free space between the piston rod an the piston when the button and the push rod is in their not operated position.
The size of set doses is mainly indicated by a pointer or through a window which is coupled to one of the dose setting elements and indicates a number corresponding to the set dose on a scale coupled to the other dose setting element the size of the dose being proportional with the distance the two elements have been moved relative to each other. This fact sets a limit for the size of the figures on the scale although different initiatives has been taken to expand the space available for these figures. The dose setting is performed by turning a screw or a wheel or by rotating two parts of the injection device relative to each other and currently monitor the size of the dose set. By some constructions of the dose setting mechanism it is not possible to reduce the set dose if the two dose setting elements have been moved too far in relation to each other, i.e. if a too large dose has been set. By other constructions it is necessary to make sure that the device is reset before a dose can be set. A device which is very simple to use is aimed at. The simplest form of device should only show a very simple dose setting device and an injection button.